The present invention relates to semiconductor laser diodes, and particularly to high power laser diodes.
High power laser diodes are important components in the technology of optical communication, particularly because such laser diodes can be used for fiber pumping and other high power laser diode applications. This allows all-optical fibre communication systems to be designed, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as the reliability of such systems. Other uses of such high power laser diodes include cable TV (CATV) amplifiers, printing applications, and medical applications.
A schematic representation of a typical semiconductor laser diode 100 is shown in FIG. 1, and consists of a (strained) quantum well active region 103 sandwiched between two semiconductor (AlGaAs) cladding layers 104, 105. A first cladding layer 104, which is grown first on a substrate, is commonly referred to as a lower cladding layer, and is typically n-type doped. A second cladding layer 105, which is grown second on the substrate, after growth of the active region 103, is commonly referred to as an upper cladding layer, and is typically p-type doped. The laser diode 100 includes two electrodes 101, 102. A first electrode metallization layer (n-metallisation layer) 101 provides electrical contact to the first cladding layer 104, and a second electrode metallization layer (p-metallisation layer) 102 provides electrical contact to the second cladding layer 105.
It has been demonstrated that defects generated during operation, often at the output end (usually known as the front end or front facet) 106 of the laser diode 100, are responsible for the failure of the laser diode. This is believed to be due to an inherent non-uniform current density distribution along a laser cavity, with the highest current flowing towards the front end of the laser diode. Non-uniform photon density distribution within the laser cavity is responsible for this non-uniform current density. FIG. 2 illustrates photon density distribution 200, 201, 202 for some high-power laser diodes. This ideal case can exist if the series resistance of the laser diode is zero. In this case the current distribution should be in line with the photon density distribution. It can therefore be understood from FIG. 2 that the current density towards the front end of the laser diode could be as much as 20 times higher than towards the back end.
Such high current density can result in local overheating of the laser diode, which is also responsible for the predominant defect formation at the front end. This local heating has a secondary adverse effect on reliability.
It has been further demonstrated that finite internal resistance of the laser diode 100 may mitigate to some extent the effect of the current non-uniformity. Distributed series resistance is normally formed within the p-cladding layer 105 of the laser diode 100. The voltage drop due to the series resistance provides a negative feedback for the current distribution along an axis of the laser diode 100. However, increasing the series resistance of the laser diode affects some operational characteristics. For example, this affects negatively the conversion efficiency. In addition, increasing internal series resistance, i.e. by lowering the carrier concentration in the layers of the laser diode 100, increases heat generation within low thermal conductivity materials of the laser diode.
WO 2007/000615 discloses a high power laser diode in which current injection towards the front end of the diode is reduced by using different types of electrical connection attached to the metallisation layer. The electrical connections are shaped or constructed by “wire-bonding”. The location of the wire bonded electrical connections is chosen so as to reduce the current injection at the front end of the device.
Various other wire-bonding configurations have been tried, but modifying wire-bonding configuration on its own has been found inadequate to provide uniform current distribution. Other techniques such as submount profiling have also been tried, but proved too weak to fully compensate the longitudinal non-uniformity of the current distribution.
Thus, there is a need for another way of producing uniform current distribution towards the front end of the laser diode in order to improve reliability.